The present invention relates to probe-head construction for a coordinate-measuring instrument, wherein the probe head relies on self-contained electrical-switch means, in connection with assuring positioned reference for each measurement, and for other measuring purposes. Such probe heads are described, for example, in Federal Republic of Germany Pat. Nos. 2,347,633 and 2,712,181, and in Federal Republic of Germany OS No. 2,743,665; the corresponding U.S. patents for these German cases are U.S. Pat. Nos. 4,177,568, 4,153,998, and 4,136,458, respectively. In the known electrically switching probe heads, the mounting or seating location from which the probe pin lifts off, upon workpiece contact in the course of the probing process, is generally an isostatic three-point mounting formed of, e.g., paired balls; the paired balls of all three mounting points are connected as contact pairs in an electric ciccuit which is closed only when three electrically conductive mating members on the probe pin are fully seated against the respective ball pairs, to close all three switches and thus to electrically certify the at-rest position of the probe pin. The switching-contact feature is needed, for example, to supply machine-control means of the coordinatemeasuring instrument with trigger pulses for storing of length-measurement values at the time of work-contact. Furthermore, this switch-contact feature is used to report whether the probe pin has again returned to its seated reference position after completing each work-contacting procedure, it being understood that such seating determines the calibrated-zero position of the work-contact ball and therefore establishes that the probe head is ready for the next measurement.
It has now been found that, with an error frequency of about once every several thousand work-contact cycles, all three switch-contacts do not close upon termination of a work-contacting cycle. This error does not have any systematic cause but occurs due to the fact that the probe pin does not return entirely and precisely into its zero position, due to residual mechanical friction in its threepoint mounting. This frequency of error is, to be sure, very small, but in certain types of coordinate-measuring instrument operation, this error, i.e., a switch contact remaining open after a work-contact, has serious consequences.
Coordinate-measuring instruments are being used to an increasing extent in multiple-measurement operations and measure, for example during an unmanned night shift under CNC control (i.e., without attendants or operating personnel), a large number of workpieces which are clamped on a measurement table or fed continuously via a handling system. If such an error occurs in this type of operation, then the machine control stops the CNC operation for reasons of safety, and the coordinate-measuring instrument remains shut down until the beginning of the following shift, at which time the error can be noted and manually eliminated. It is clear that such interruptions in operation are extremely undesirable.